The present invention relates to a rack-pinion type power steering apparatus and, more particularly, to a rack-pinion type power steering apparatus having a rotary type channel selector valve (rotary valve).
A rack-pinion type power steering apparatus is advantageous in that it is lightweight, has a comparatively simple arrangement, is excellent in steering performance, and requires a small space when being incorporated in a vehicle, and is accordingly among those which are often employed as a power steering apparatus. Although apparatuses having various structures are conventionally proposed as such a rack-pinion type power steering apparatus, they still have room for improvement in achieving downsizing and weight reduction of a vehicle and making the apparatus compact.
For example, each of Japanese Utility Model Laid-Open Nos. 55-114763 and 55-114764 discloses a structure in which a sleeve constituting the outer valve member of a rotary type channel selector valve is integrally connected to and formed on a pinion shaft having pinion teeth that mesh with the rack teeth of a rack in order to simplify the arrangement of the entire apparatus, reduce the number of constituent components, and further decrease the size of the entire apparatus and to simplify the working and assembly operations.
In the former conventional case, a pinion shaft integrally connected to a sleeve is axially supported by a steering body with rolling bearings, e.g., a ball bearing and a needle bearing, formed on the two end portions of the pinion shaft. The valve housing portion can be integrally formed with the steering body. A stub shaft having a rotor pivotally disposed on the inner circumferential portion of the sleeve is axially supported by a ball bearing held by a plug member disposed at the open end of the valve housing.
In the latter conventional case, the two end portions of the pinion teeth of the pinion shaft are axially supported by a steering body with rolling bearings, e.g., a ball bearing and a needle bearing. A stub shaft connected to a rotor is pivotally supported by a ball bearing held by a body portion (although this body portion is shown as an integral member in the drawings, it is divisionally formed in practice) which is integral with the steering body.
In a conventional rack-pinion type power steering apparatus, a pinion shaft and a sleeve constituting the outer valve member of a rotary type channel selector valve are integrally formed, as described above. However, the conventional apparatus with this structure has a problem in the structure wherein the stub shaft, having a rotor inserted in the sleeve and relatively, rotatably displaceable, is axially supported in the steering body. Therefore, the conventional apparatus is not sufficient in achieving a smooth valve action, and it is desired that any countermeasure be taken.
This problem will be described in more detail. In the conventional case described above, the stub shaft having the rotor is pivotally, axially supported through a plug member engaging with the steering body or a bearing, e.g., a ball bearing, directly held by the body, not through the axial support portion that axially supports the pinion shaft integrally formed with the sleeve on the steering body. In order to obtain a smooth valve action with this arrangement, very high axial support precision is required.
In practice, however, it is impossible to cope with this demand. More specifically, when this structure is employed, the bearing axially supporting the stub shaft must have a bearing gap smaller than the valve clearance and must maintain strict concentricity allowed as it is formed on a member separate from the sleeve. It is almost impossible to achieve this demand.
In particular, in the rotary type channel selector valve arranged between the sleeve and rotor described above, the rotor integrally connected to the stub shaft of the steering wheel and the sleeve integrally connected to the pinion shaft of the steering wheel must be combined such that they can concentrically, rotatably displace relative to each other and be disposed in the valve housing. This is because the oil pump serving as the fluid pressure generating source, the oil tank, the inlet port and the return port communicating with the left and right cylinder chambers constituting the power cylinder, and the channels serving as the right and left output ports are connected to each other by selectively connecting and disconnecting a plurality of channel grooves radially formed in the outer circumferential surface of the rotor and the inner circumferential surface of the sleeve to and from each other by relative rotational displacement between the rotor and sleeve, thereby selecting the channels of the fluid pressure circuit (hydraulic circuit).
In the conventional rack-pinion type power steering apparatus described above, when the sleeve is integrally connected to the pinion shaft, it is difficult to obtain a smooth rotational displacement between the sleeve and the rotor integrally connected to the stub shaft, and a smooth valve action is hindered accordingly. A countermeasure that can solve this problem is therefore sought for.